Preparation of isoprene feedstock



y 1968 w. R. EDWARDS E AL PREPARATION OF ISOPRENE FEEDSTOCK Filed April 1. 1963 I46 METHYL- I-PENTENE FRACTIONATION 3 rowan '22 ansmu: muse I46 v |44 CATALYTIC POLWER NAPHTHA sxmacnpu n m 2 lONE 0 SETITBLER 1 0-0 B y 34 serum FAT Aclo is v PHASE H4 '30 I08 no, 7

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WILLIAM R. EDWARDS, ROBERT D. WESSELHOFT, BERT B. WILLIAMS,

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ATTORNEY- United States Patent 3,382,289 PREPARATION OF ISOPRENE FEEDSTOCK William R. Edwards and Robert D. Wesselhoft, Baytown, Tex., and Bert B. Williams, Princeton, N.J., assignors, by mesne assignments, to Esso Research and Engineering Company, Elizabeth, N.J., a corporation of Delaware Filed Apr. 1, 1963, Ser. No. 269,438 1 Claim. (Cl. 260680) The present invention relates to the preparation of a feedstock suitable for pyrolitic cracking in producing isoprene. More particularly, the present invention is directed to a method of obtaining a pyrolysis feedstock which contains an optimum amount of isoprene precursors. Specifically, the present invention relates to a process whereby a hydrocarbon stream (such as catalytic naphtha) which contains both isoprene precursors and nonprecursors is contacted with sulfuric acid to selectively isomerize at least a portion of the nonprecursors into isoprene-forming compounds, while separating the resulting aggregate of precursors from the remaining nonprecursors. Thus, a pyrolysis feedstock may be prepared which optimizes the production of isoprene from the initial hydrocarbon stream.

Isoprene, which has the structural formula:

is produced by the pyrolysis of certain tertiary olefins, among them 2-methyl-2-pentene, 3-methyl-2-pentene, and Z-ethylbutene. It cannot be obtained economically by the pyrolysis of 2-methyl-1-pentene, 4-methyl-2-pentene, nhexene, or 3- and 4-rnethyl-l-pentene, which pyrolyze preponderantly to gases. This results in a net loss of feedstock and places a nonproductive burden on the pyrolysis and separation facilities in the isoprene unit, so that larger capital investment is required per unit of isoprene produced.

Thus, it is important to provide a feedstock to the pyrolysis zone which contains a maximum amount of isoprene precursors in order to reduce the load on the pyrolysis zone and fractionation facilities, and to prevent the loss of nonprecursors which would be valuable for use elsewhere rather than being reduced to the less valuable gases in the plyrolysis zone. In summary, the problem is to obtain the most isoprene product from catalytic naphtha at the least cost.

The problem may best be understood by referring to catalytic naphtha as a hydrocarbon source stream. A typical catalytic naphtha would include a number of hydrocarbons, the principal ones being shown below.

Cis-3-methyl-2-pentene 154 3,382,289 Patented May 7, 1968 t-Hexene-Z 154 n-Hexane 156 CiS-heXene-Z 156 t-3-methyl-2-pentene 159 Methylcyclopentane 161 2,3-dimethyl-2-butene 164 4-rnethylcyclopentene 167 One approach to the separation of isoprene precursors from the catalytic naphtha would involve a precise fractionation of that catalytic naphtha to obtain a pyrolysis feed boiling within the range of F. to F. This stream would include the isoprene precursors but would also include nonprecursors such as n-hexenes which, although mounting to only about 2% of the initial catalytic naphtha stream, would constitute between 20% and 30% of the pyrolysis feed. Obvously, this approach is not suitable.

If the precise fractionation were followed by sulfuric 'acid extraction in order to separate the precursors from the nonprecursors, only about 5% of the total catalytic naphtha feed would be recovered as the pyrolysis feed. This also leaves much to be desired.

Surprisingly, by taking a wide range fractionation product of the catalytic naphtha (that is, the entire C portion), and extracting some of the nonprecursors as well as all of the isoprene precursors into sulfuric acid, it has been found that the tertiary olefin nonprecursors, such as Z-met'hyl-l-pentene, are isomerized into isoprene precursors, resulting in a higher mol quantity of isoprene precursors in the pyrolysis feed than were initially present in the catalytic naphtha.

Thus, several advantages are obtained. Firstly, the initial fractionation step need not be as precise as would be expected from the direct approach, which means that a small investment in the initial distillation facilities is required; and, secondly, the amount of isoprene percursors obtained from a given volume of catalytic naphtha is increased, the amount of isoprene precursors in the pyrolysis feedstock amounting to about 150% of the amount recoverable by the more precise fractionation method.

Considering the process in its specific aspects, any hydrocarbon stream which contains C olefins may be used as a source stream for the present process, so long as isoprene precursors (such as 2-methyl-2-pentene, 3-methyl- Z-pentcne, 2,3-dimethyl-2-butene, and Z-ethyl-l-butene) and nonprecursors (such as 2-methyl-1-pentene and 3- methyl-l-pcntene) are present in commercially attractive quantities. The stream may boil roughly within the range of 100 F. to 200 F. Suitable streams are those obtained from catalytic cracking, thermal cracking, acid catalyzed dimerization of propylene, etc. 1

The initial fractionation step is chosen to obtain a rough heart out boiling within the range of 125 F. to R, which not only contains the isoprene precursors but also contains the nonprecursors which are to be isomerized.

The resulting rough heart out is then contacted with 60 to 75 weight percent sulfuric acid at a temperature within the range of 20 F. to 60 F., with an acid-to-hydrocarbon ratio within the range of 10:1 to 1:10. The contact time is a function of temperature, acid strength, and mixing eificiency, etc., but broadly may fall within the period from 5 minutes to 3 hours. This determination is most easily based on the hydrocarbon phase.

This contact produces a fat acid phase, which contains from 15% to 35% of reacted olefins. These are both precursors and isomerizable nonprecursors of isoprene. The extract phase may be easily separated from the remainder of the heart out (which constitutes the rafiinate phase) by decantation or vacuum flashing, or both. While the olefins are in the fat acid phase, the nonprecursors are isomerized by the catalytic action of the acid, and become valuable as isoprene precursors. Thus, the sulfuric acid acts in two beneficial ways:

(1) It separates isoprene precursors from the main body of nonprecursors; and

(2) 1t isomerizes at least a portion of the nonprecursors into the desired isoprene precursors.

The fat acid phase then contains the olefins which are desired as a pyrolysis feedstock. These olefins are released from the acid by any of the wellknown so-called regeneration methods, either with or without dilution of that acid phase. Suitable methods comprise re-extraction with a solvent hydrocarbon, direct heating with steam, indirect heating, etc. The preferred method involves thermal regeneration which involves heating the fat acid phase indirectly and without dilution to a final temperature of from 150 F. to 250 F., at a minimum rate of about 400 F. per minute. The residence time in the heating zone may be from 0.001 second to a suitable upper time limit such as about minutes. This rapid heating breaks the fat acid phase into two phases, a lean acid phase and an olefin phase. Polymer losses are minimized by the high rate of heating.

The two phases are easily separated by settling and decantation, the olefins being withdrawn as the upper phase. The olefin phase may then be fractionated to obtain a pyrolysis feed which is almost entirely made up of isoprene precursors, or (since the amount of nonprecursors is minimal) may be charged directly to the pyrolysis zone without fractionation.

The pyrolysis reaction is suitably accomplished by heating the pyrolysis feedstock at a temperature of about 800 C. (1472 F.) for 0.01 to 0.05 second, but the particular manner of pyrolysis forms no part of the present invention.

All of this will be seen by avertence to the drawing wherein a schematic flow diagram of the invention is set forth.

Referring now to the drawing, the catalytic naphtha is introduced into the column by way of line 102, and a rough heart cut fraction is removed by way of line 104. The lighter material is removed by way of line 106 whereas the heavy boiling material is removed by way of line 108. The rough heart cut may suitably boil within a range of 125 F. to about 165 F., and includes a certain amount of 2,3-dirnethylbutene-2. The C catalytic naphtha is then contacted in the extraction zone 110 with sulfuric acid introduced by way of line 112, and is intimately admixed therewith by means 114.

The admixed hydrocarbon and acid are withdrawn by way of line 116 and are passed into settler 118 from which is withdrawn a hydrocarbon raffinate phase by way of line 120 and a fat acid extract phase by way of line 122.

As is indicated in the drawing, the fat acid stream is conducted by way of line 122 into a heater 124 wherein the fat acid extract is regenerated. An acid phase and a tertiary olefin phase are produced by this regeneration and the two phases are conducted by way of line 126 into a settler 128, from which the acid phase is withdrawn by way of line 130 and recycled into the extraction zone 110.

The tertiary olefin phase, which comprises not only the isoprene precursors which were resent in the catalytic naphtha initially, but also isoprene precursors which have been formed by isomerization within the sulfuric acid phase of 2,3-dimethyl-1-butene and 2-metliyl-1-pentene into 2,3-dimethyl-2-butene and 2-methyl-2-pentene, are then passed by Way of lines 132 and 134 into a pyrolysis zone 136, wherein the precursors are pyrolyzed at a temperature of about 800 C. for a time period of about 0.01 to 0.05 second. The pyrolysis products are withdrawn by Way of line 138 and passed to fractionation facilities not shown, from which the isoprene is recovered.

Alternatively, since some of the 2-methyl-l-pentene may remain unisomerized in the tertiary olefin phase removed by way of line 132, the tertiary olefin phase may be passed by way of lines 132 and into a fractionation tower 142, from whence the polymer is discharged by way of line 144 and Z-methyl-l-pentene is recycled by Way of line 146 into the extraction zone 110, wherein it may be further isomerized into Z-methyl-Z-pentene. The tertiary olefin phase may then be withdrawn by way of line 148, and this phase will comprise almost entirely isoprene precursors.

Thus, it is seen that by following the above-set-forth steps, the nonpreeursors in catalytic naphtha are converted into isoprene precursors, and to obtain a pyrolysis feedstock containing more isoprene precursors than were present in the catalytic naphtha feedstock.

Example 1 In order to illustrate the advantages which are incumbent in the practice of the present invention, a comparison of the amount of the various isoprene precursors and nonprecursors which are present in a C catalytic naphha and in the pyrolysis feedstock which would be obtained therefrom by the practice of the present invention and by the use of a more precise first fractionation are set forth in Table II below.

TABLE II Feed Fraction Extraction Product isoprene Feed Mol percent in Mel percent, Molpcrecnt, Mols After Mols After B.P., F Ca-430" I l50160 F. 125-165" I Extraction, Extraction, M Mnls,

Cat. Naphtha Cut ut 160 F. 125-165 I ISO- F 125-165 F Cut Out Cut Out 2,2-dimethylhutane.

3-methyl-1-pcntene. 129 34 1. 3 4methyl-1-pcntene. 129 42 1. 5 2,3-dimetltyl-1-butcn 132 60 2. 2 cis-4-methyl-2-pentene 133 30 1. 1 2,3-dimethylbutane 136 .27 1.0 t-4-methyl-2-pentene- 137 90 3. 3 Z-methylpentane 140 3. 00 10. 9 2-mothyl-1-pentene. 141 2. 33 8. 6 3-methylpentane... 146 1. 72 6. 3 l-hoxene 14G 27 1.0 z-etliyl-l-butene 148 79 4. 6 2. 9 3-rnethylcyclopentcne 149 2. 70 15. (3 9. 8 eis-3-hexene 152 33 1. 9 1. 2 t-3-hexene 153 43 2. 5 1. 6 z-methyl-ilpenten 153 2. 62 15. 3 9. 6 cis-S-methylQ-pcnte 154 2. 67 15. G 9. 8 t-hcxenc-2 154 81 4. 8 3.0 n-hexane 156 1. 39 8. 1 5.1 cis-IIexenc-Z 156 76 4. 5 2. 8 t-3-mcthyL2-pentene. 159 1. 78 10. 4 6. 5 Methyleyclopentane. 161 2. 16 12. G 7. 9 2,3-dimethyl-2-butene 164 71 4. 1 2. 6 t-methylcyclopentene 1G7 Total i7. 30 100. 0 100, O

By advertence to Table II it is seen that the total mols of isoprene precursors in the pyrolysis feedstock obtained by precise fractionation amounts to 21.4 mols per 100 mols of catalytic naphtha, whereas, by the practice of the present invention, 29.0 m-ols are obtained. Since the isoprene is produced from the precursors, it is apparent that almost a advantage in isoprene produced per unit of catalytic naphtha feed may be obtained by the use of the present invention.

Having set forth a preferred mode of the practice of the present invention, what is intended to be covered by the Letters Patent should be limited not by the specific examples herein given, but rather only by the appended claim.

We claim: 1. A method of producing isoprene from a catalytic naphtha which comprises fractionating said catalytic naphtha to obtain a heart out boiling over the range of about 125 F. to about 165 E,

which contains 2,3-dirnethyl-1-butene, 2-methyl-1-pentene, 2-ethyl-1-butene, 2-methyl-2-pentene, 3-methyl- 2-pentene, and 2,3-dimethyl-2-butene,

contacting said heart cut with to sulfuric acid at a temperature within the range of 20 F. to 60 F. for a time within the range of 5 minutes to 3 hours and at an acid-to-hydrocarbon ratio within the range of 10:1 to 1:10 to obtain a fat acid extract recycling said Z-methyl-l-pentene stream to said contacting zone,

and pyrolyzing said product stream at about 1472 F.

for about 0.01 to 0.05 second to obtain isoprene in high yield.

References Cited UNITED STATES PATENTS 2,391,555 12/1945 de Simo et al. 260-680 2,404,056 7/1946 Gorin et al. 260-6 3,104,269 9/1963 Schatfel 260-680 3,129,265 4/1964 Edwards et al. 260-677 3,150,200 9/1964 Edwards et al. 260-677 3,173,968 3/1965 Edwards et al. 260-6832 FOREIGN PATENTS 832,475 4/1960 Great Britain.

PAUL M. COUGHLLAN, JR., Primary Examiner. 

1. A METHOD OF PRODUCING ISOPRENE FROM A CATALYTIC NAPHTHA WHICH COMPRISES FRACTIONATING SAID CATALYTIC NAPHTHA TO OBTAIN A HEART CUT BOILING OVER THE RANGE OF ABOUT 125*F. TO ABOUT 165*F., WHICH CONTAINS 2.3-DIMETHYL-1-BUTENE, 2-METHYL-1-PENTENE, 2-ETHYL-1-BUTENE, 2-METHYL-2-PENTENE, 3-METHYL2-PENTENE, AND 2,3-DIMETHYL-2-BUTENE, CONTACTING SAID HEART CUT WITH 60% TO 75% SULFURIC ACID AT A TEMPERATURE WITHIN THE RANGE OF 20*F. TO 60*F. FOR A TIME WITHIN THE RANGE OF 5 MINUTES TO 3 HOURS AND AT AN ACID-TO-HYDROCARBON RATIO WITHIN THE RANGE OF 10:1 TO 1:10 TO OBTAIN A FAT ACID EXTRACT PHASE CONTAINING FROM 15% TO 35% REACTED HYDROCARBONS AND A RAFFINATE PHASE, SEPARATING SAID EXTRACT PHASE FORM SAID RAFFINATE PHASE, INDIRECTLY HEATING SAID EXTRACT PHASE AT A RATE OF AT LEAST 400*F. PER MINUTE TO A FINAL TEMPERATURE WITHIN THE RANGE OF 150*F. TO 250*F. TO RELEASE THE OLEFINS FROM COMBINATION WITH THE ACID AND OBTAIN AN OLEFIN PHASE AND AN ACID PHASE, FRACTIONATING SAID OLEFIN PHASE TO OBTAIN A 2-METHYL-1PENTENE STREAM AND A PRODUCT STREAM, RECYCLING SAID 2-METHYL-1-PENTENE STREAM TO SAID CONTACTING ZONE, AND PYROLYZING SAID PRODUCT STREAM AT ABOUT 1472*F. FOR ABOUT 0.01 TO 0.05* SECOND TO OBTAIN ISOPRENE IN HIGH YIELD. 